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</pre><pre class="rust ">
<span class="comment">// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 &lt;LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0&gt; or the MIT license</span>
<span class="comment">// &lt;LICENSE-MIT or http://opensource.org/licenses/MIT&gt;, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>

<span class="comment">// This module implements the Pike VM. That is, it guarantees linear time</span>
<span class="comment">// search of a regex on any text with memory use proportional to the size of</span>
<span class="comment">// the regex.</span>
<span class="comment">//</span>
<span class="comment">// It is equal in power to the backtracking engine in this crate, except the</span>
<span class="comment">// backtracking engine is typically faster on small regexes/texts at the</span>
<span class="comment">// expense of a bigger memory footprint.</span>
<span class="comment">//</span>
<span class="comment">// It can do more than the DFA can (specifically, record capture locations</span>
<span class="comment">// and execute Unicode word boundary assertions), but at a slower speed.</span>
<span class="comment">// Specifically, the Pike VM exectues a DFA implicitly by repeatedly expanding</span>
<span class="comment">// epsilon transitions. That is, the Pike VM engine can be in multiple states</span>
<span class="comment">// at once where as the DFA is only ever in one state at a time.</span>
<span class="comment">//</span>
<span class="comment">// Therefore, the Pike VM is generally treated as the fallback when the other</span>
<span class="comment">// matching engines either aren&#39;t feasible to run or are insufficient.</span>

<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;

<span class="kw">use</span> <span class="ident">exec</span>::<span class="ident">ProgramCache</span>;
<span class="kw">use</span> <span class="ident">input</span>::{<span class="ident">Input</span>, <span class="ident">InputAt</span>};
<span class="kw">use</span> <span class="ident">prog</span>::{<span class="ident">Program</span>, <span class="ident">InstPtr</span>};
<span class="kw">use</span> <span class="ident">re_trait</span>::<span class="ident">Slot</span>;
<span class="kw">use</span> <span class="ident">sparse</span>::<span class="ident">SparseSet</span>;

<span class="doccomment">/// An NFA simulation matching engine.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Fsm</span><span class="op">&lt;</span><span class="lifetime">&#39;r</span>, <span class="ident">I</span><span class="op">&gt;</span> {
    <span class="doccomment">/// The sequence of opcodes (among other things) that is actually executed.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// The program may be byte oriented or Unicode codepoint oriented.</span>
    <span class="ident">prog</span>: <span class="kw-2">&amp;</span><span class="lifetime">&#39;r</span> <span class="ident">Program</span>,
    <span class="doccomment">/// An explicit stack used for following epsilon transitions. (This is</span>
    <span class="doccomment">/// borrowed from the cache.)</span>
    <span class="ident">stack</span>: <span class="kw-2">&amp;</span><span class="lifetime">&#39;r</span> <span class="kw-2">mut</span> <span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">FollowEpsilon</span><span class="op">&gt;</span>,
    <span class="doccomment">/// The input to search.</span>
    <span class="ident">input</span>: <span class="ident">I</span>,
}

<span class="doccomment">/// A cached allocation that can be reused on each execution.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Cache</span> {
    <span class="doccomment">/// A pair of ordered sets for tracking NFA states.</span>
    <span class="ident">clist</span>: <span class="ident">Threads</span>,
    <span class="ident">nlist</span>: <span class="ident">Threads</span>,
    <span class="doccomment">/// An explicit stack used for following epsilon transitions.</span>
    <span class="ident">stack</span>: <span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">FollowEpsilon</span><span class="op">&gt;</span>,
}

<span class="doccomment">/// An ordered set of NFA states and their captures.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="kw">struct</span> <span class="ident">Threads</span> {
    <span class="doccomment">/// An ordered set of opcodes (each opcode is an NFA state).</span>
    <span class="ident">set</span>: <span class="ident">SparseSet</span>,
    <span class="doccomment">/// Captures for every NFA state.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// It is stored in row-major order, where the columns are the capture</span>
    <span class="doccomment">/// slots and the rows are the states.</span>
    <span class="ident">caps</span>: <span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">Slot</span><span class="op">&gt;</span>,
    <span class="doccomment">/// The number of capture slots stored per thread. (Every capture has</span>
    <span class="doccomment">/// two slots.)</span>
    <span class="ident">slots_per_thread</span>: <span class="ident">usize</span>,
}

<span class="doccomment">/// A representation of an explicit stack frame when following epsilon</span>
<span class="doccomment">/// transitions. This is used to avoid recursion.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="kw">enum</span> <span class="ident">FollowEpsilon</span> {
    <span class="doccomment">/// Follow transitions at the given instruction pointer.</span>
    <span class="ident">IP</span>(<span class="ident">InstPtr</span>),
    <span class="doccomment">/// Restore the capture slot with the given position in the input.</span>
    <span class="ident">Capture</span> { <span class="ident">slot</span>: <span class="ident">usize</span>, <span class="ident">pos</span>: <span class="ident">Slot</span> },
}

<span class="kw">impl</span> <span class="ident">Cache</span> {
    <span class="doccomment">/// Create a new allocation used by the NFA machine to record execution</span>
    <span class="doccomment">/// and captures.</span>
    <span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">_prog</span>: <span class="kw-2">&amp;</span><span class="ident">Program</span>) <span class="op">-&gt;</span> <span class="self">Self</span> {
        <span class="ident">Cache</span> {
            <span class="ident">clist</span>: <span class="ident">Threads</span>::<span class="ident">new</span>(),
            <span class="ident">nlist</span>: <span class="ident">Threads</span>::<span class="ident">new</span>(),
            <span class="ident">stack</span>: <span class="macro">vec</span><span class="macro">!</span>[],
        }
    }
}

<span class="kw">impl</span><span class="op">&lt;</span><span class="lifetime">&#39;r</span>, <span class="ident">I</span>: <span class="ident">Input</span><span class="op">&gt;</span> <span class="ident">Fsm</span><span class="op">&lt;</span><span class="lifetime">&#39;r</span>, <span class="ident">I</span><span class="op">&gt;</span> {
    <span class="doccomment">/// Execute the NFA matching engine.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// If there&#39;s a match, `exec` returns `true` and populates the given</span>
    <span class="doccomment">/// captures accordingly.</span>
    <span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">exec</span>(
        <span class="ident">prog</span>: <span class="kw-2">&amp;</span><span class="lifetime">&#39;r</span> <span class="ident">Program</span>,
        <span class="ident">cache</span>: <span class="kw-2">&amp;</span><span class="ident">ProgramCache</span>,
        <span class="ident">matches</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="ident">bool</span>],
        <span class="ident">slots</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="ident">Slot</span>],
        <span class="ident">quit_after_match</span>: <span class="ident">bool</span>,
        <span class="ident">input</span>: <span class="ident">I</span>,
        <span class="ident">start</span>: <span class="ident">usize</span>,
    ) <span class="op">-&gt;</span> <span class="ident">bool</span> {
        <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">cache</span> <span class="op">=</span> <span class="ident">cache</span>.<span class="ident">borrow_mut</span>();
        <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">cache</span> <span class="op">=</span> <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">cache</span>.<span class="ident">pikevm</span>;
        <span class="ident">cache</span>.<span class="ident">clist</span>.<span class="ident">resize</span>(<span class="ident">prog</span>.<span class="ident">len</span>(), <span class="ident">prog</span>.<span class="ident">captures</span>.<span class="ident">len</span>());
        <span class="ident">cache</span>.<span class="ident">nlist</span>.<span class="ident">resize</span>(<span class="ident">prog</span>.<span class="ident">len</span>(), <span class="ident">prog</span>.<span class="ident">captures</span>.<span class="ident">len</span>());
        <span class="kw">let</span> <span class="ident">at</span> <span class="op">=</span> <span class="ident">input</span>.<span class="ident">at</span>(<span class="ident">start</span>);
        <span class="ident">Fsm</span> {
            <span class="ident">prog</span>: <span class="ident">prog</span>,
            <span class="ident">stack</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">cache</span>.<span class="ident">stack</span>,
            <span class="ident">input</span>: <span class="ident">input</span>,
        }.<span class="ident">exec_</span>(
            <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">cache</span>.<span class="ident">clist</span>,
            <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">cache</span>.<span class="ident">nlist</span>,
            <span class="ident">matches</span>,
            <span class="ident">slots</span>,
            <span class="ident">quit_after_match</span>,
            <span class="ident">at</span>,
        )
    }

    <span class="kw">fn</span> <span class="ident">exec_</span>(
        <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>,
        <span class="kw-2">mut</span> <span class="ident">clist</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">Threads</span>,
        <span class="kw-2">mut</span> <span class="ident">nlist</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">Threads</span>,
        <span class="ident">matches</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="ident">bool</span>],
        <span class="ident">slots</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="ident">Slot</span>],
        <span class="ident">quit_after_match</span>: <span class="ident">bool</span>,
        <span class="kw-2">mut</span> <span class="ident">at</span>: <span class="ident">InputAt</span>,
    ) <span class="op">-&gt;</span> <span class="ident">bool</span> {
        <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">matched</span> <span class="op">=</span> <span class="bool-val">false</span>;
        <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">all_matched</span> <span class="op">=</span> <span class="bool-val">false</span>;
        <span class="ident">clist</span>.<span class="ident">set</span>.<span class="ident">clear</span>();
        <span class="ident">nlist</span>.<span class="ident">set</span>.<span class="ident">clear</span>();
<span class="lifetime">&#39;LOOP</span>:  <span class="kw">loop</span> {
            <span class="kw">if</span> <span class="ident">clist</span>.<span class="ident">set</span>.<span class="ident">is_empty</span>() {
                <span class="comment">// Three ways to bail out when our current set of threads is</span>
                <span class="comment">// empty.</span>
                <span class="comment">//</span>
                <span class="comment">// 1. We have a match---so we&#39;re done exploring any possible</span>
                <span class="comment">//    alternatives. Time to quit. (We can&#39;t do this if we&#39;re</span>
                <span class="comment">//    looking for matches for multiple regexes, unless we know</span>
                <span class="comment">//    they all matched.)</span>
                <span class="comment">//</span>
                <span class="comment">// 2. If the expression starts with a &#39;^&#39; we can terminate as</span>
                <span class="comment">//    soon as the last thread dies.</span>
                <span class="kw">if</span> (<span class="ident">matched</span> <span class="op">&amp;&amp;</span> <span class="ident">matches</span>.<span class="ident">len</span>() <span class="op">&lt;=</span> <span class="number">1</span>)
                    <span class="op">||</span> <span class="ident">all_matched</span>
                    <span class="op">||</span> (<span class="op">!</span><span class="ident">at</span>.<span class="ident">is_start</span>() <span class="op">&amp;&amp;</span> <span class="self">self</span>.<span class="ident">prog</span>.<span class="ident">is_anchored_start</span>) {
                    <span class="kw">break</span>;
                }

                <span class="comment">// 3. If there&#39;s a literal prefix for the program, try to</span>
                <span class="comment">//    jump ahead quickly. If it can&#39;t be found, then we can</span>
                <span class="comment">//    bail out early.</span>
                <span class="kw">if</span> <span class="op">!</span><span class="self">self</span>.<span class="ident">prog</span>.<span class="ident">prefixes</span>.<span class="ident">is_empty</span>() {
                    <span class="ident">at</span> <span class="op">=</span> <span class="kw">match</span> <span class="self">self</span>.<span class="ident">input</span>.<span class="ident">prefix_at</span>(<span class="kw-2">&amp;</span><span class="self">self</span>.<span class="ident">prog</span>.<span class="ident">prefixes</span>, <span class="ident">at</span>) {
                        <span class="prelude-val">None</span> <span class="op">=&gt;</span> <span class="kw">break</span>,
                        <span class="prelude-val">Some</span>(<span class="ident">at</span>) <span class="op">=&gt;</span> <span class="ident">at</span>,
                    };
                }
            }

            <span class="comment">// This simulates a preceding &#39;.*?&#39; for every regex by adding</span>
            <span class="comment">// a state starting at the current position in the input for the</span>
            <span class="comment">// beginning of the program only if we don&#39;t already have a match.</span>
            <span class="kw">if</span> <span class="ident">clist</span>.<span class="ident">set</span>.<span class="ident">is_empty</span>()
                <span class="op">||</span> (<span class="op">!</span><span class="self">self</span>.<span class="ident">prog</span>.<span class="ident">is_anchored_start</span> <span class="op">&amp;&amp;</span> <span class="op">!</span><span class="ident">all_matched</span>) {
                <span class="self">self</span>.<span class="ident">add</span>(<span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">clist</span>, <span class="ident">slots</span>, <span class="number">0</span>, <span class="ident">at</span>);
            }
            <span class="comment">// The previous call to &quot;add&quot; actually inspects the position just</span>
            <span class="comment">// before the current character. For stepping through the machine,</span>
            <span class="comment">// we can to look at the current character, so we advance the</span>
            <span class="comment">// input.</span>
            <span class="kw">let</span> <span class="ident">at_next</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">input</span>.<span class="ident">at</span>(<span class="ident">at</span>.<span class="ident">next_pos</span>());
            <span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">0</span>..<span class="ident">clist</span>.<span class="ident">set</span>.<span class="ident">len</span>() {
                <span class="kw">let</span> <span class="ident">ip</span> <span class="op">=</span> <span class="ident">clist</span>.<span class="ident">set</span>[<span class="ident">i</span>];
                <span class="kw">if</span> <span class="self">self</span>.<span class="ident">step</span>(
                    <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">nlist</span>,
                    <span class="ident">matches</span>,
                    <span class="ident">slots</span>,
                    <span class="ident">clist</span>.<span class="ident">caps</span>(<span class="ident">ip</span>),
                    <span class="ident">ip</span>,
                    <span class="ident">at</span>,
                    <span class="ident">at_next</span>,
                ) {
                    <span class="ident">matched</span> <span class="op">=</span> <span class="bool-val">true</span>;
                    <span class="ident">all_matched</span> <span class="op">=</span> <span class="ident">all_matched</span> <span class="op">||</span> <span class="ident">matches</span>.<span class="ident">iter</span>().<span class="ident">all</span>(<span class="op">|</span><span class="kw-2">&amp;</span><span class="ident">b</span><span class="op">|</span> <span class="ident">b</span>);
                    <span class="kw">if</span> <span class="ident">quit_after_match</span> {
                        <span class="comment">// If we only care if a match occurs (not its</span>
                        <span class="comment">// position), then we can quit right now.</span>
                        <span class="kw">break</span> <span class="lifetime">&#39;LOOP</span>;
                    }
                    <span class="kw">if</span> <span class="self">self</span>.<span class="ident">prog</span>.<span class="ident">matches</span>.<span class="ident">len</span>() <span class="op">==</span> <span class="number">1</span> {
                        <span class="comment">// We don&#39;t need to check the rest of the threads</span>
                        <span class="comment">// in this set because we&#39;ve matched something</span>
                        <span class="comment">// (&quot;leftmost-first&quot;). However, we still need to check</span>
                        <span class="comment">// threads in the next set to support things like</span>
                        <span class="comment">// greedy matching.</span>
                        <span class="comment">//</span>
                        <span class="comment">// This is only true on normal regexes. For regex sets,</span>
                        <span class="comment">// we need to mush on to observe other matches.</span>
                        <span class="kw">break</span>;
                    }
                }
            }
            <span class="kw">if</span> <span class="ident">at</span>.<span class="ident">is_end</span>() {
                <span class="kw">break</span>;
            }
            <span class="ident">at</span> <span class="op">=</span> <span class="ident">at_next</span>;
            <span class="ident">mem</span>::<span class="ident">swap</span>(<span class="ident">clist</span>, <span class="ident">nlist</span>);
            <span class="ident">nlist</span>.<span class="ident">set</span>.<span class="ident">clear</span>();
        }
        <span class="ident">matched</span>
    }

    <span class="doccomment">/// Step through the input, one token (byte or codepoint) at a time.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// nlist is the set of states that will be processed on the next token</span>
    <span class="doccomment">/// in the input.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// caps is the set of captures passed by the caller of the NFA. They are</span>
    <span class="doccomment">/// written to only when a match state is visited.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// thread_caps is the set of captures set for the current NFA state, ip.</span>
    <span class="doccomment">///</span>
    <span class="doccomment">/// at and at_next are the current and next positions in the input. at or</span>
    <span class="doccomment">/// at_next may be EOF.</span>
    <span class="kw">fn</span> <span class="ident">step</span>(
        <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>,
        <span class="ident">nlist</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">Threads</span>,
        <span class="ident">matches</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="ident">bool</span>],
        <span class="ident">slots</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="ident">Slot</span>],
        <span class="ident">thread_caps</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="prelude-ty">Option</span><span class="op">&lt;</span><span class="ident">usize</span><span class="op">&gt;</span>],
        <span class="ident">ip</span>: <span class="ident">usize</span>,
        <span class="ident">at</span>: <span class="ident">InputAt</span>,
        <span class="ident">at_next</span>: <span class="ident">InputAt</span>,
    ) <span class="op">-&gt;</span> <span class="ident">bool</span> {
        <span class="kw">use</span> <span class="ident">prog</span>::<span class="ident">Inst</span>::<span class="kw-2">*</span>;
        <span class="kw">match</span> <span class="self">self</span>.<span class="ident">prog</span>[<span class="ident">ip</span>] {
            <span class="ident">Match</span>(<span class="ident">match_slot</span>) <span class="op">=&gt;</span> {
                <span class="kw">if</span> <span class="ident">match_slot</span> <span class="op">&lt;</span> <span class="ident">matches</span>.<span class="ident">len</span>() {
                    <span class="ident">matches</span>[<span class="ident">match_slot</span>] <span class="op">=</span> <span class="bool-val">true</span>;
                }
                <span class="kw">for</span> (<span class="ident">slot</span>, <span class="ident">val</span>) <span class="kw">in</span> <span class="ident">slots</span>.<span class="ident">iter_mut</span>().<span class="ident">zip</span>(<span class="ident">thread_caps</span>.<span class="ident">iter</span>()) {
                    <span class="kw-2">*</span><span class="ident">slot</span> <span class="op">=</span> <span class="kw-2">*</span><span class="ident">val</span>;
                }
                <span class="bool-val">true</span>
            }
            <span class="ident">Char</span>(<span class="kw-2">ref</span> <span class="ident">inst</span>) <span class="op">=&gt;</span> {
                <span class="kw">if</span> <span class="ident">inst</span>.<span class="ident">c</span> <span class="op">==</span> <span class="ident">at</span>.<span class="ident">char</span>() {
                    <span class="self">self</span>.<span class="ident">add</span>(<span class="ident">nlist</span>, <span class="ident">thread_caps</span>, <span class="ident">inst</span>.<span class="ident">goto</span>, <span class="ident">at_next</span>);
                }
                <span class="bool-val">false</span>
            }
            <span class="ident">Ranges</span>(<span class="kw-2">ref</span> <span class="ident">inst</span>) <span class="op">=&gt;</span> {
                <span class="kw">if</span> <span class="ident">inst</span>.<span class="ident">matches</span>(<span class="ident">at</span>.<span class="ident">char</span>()) {
                    <span class="self">self</span>.<span class="ident">add</span>(<span class="ident">nlist</span>, <span class="ident">thread_caps</span>, <span class="ident">inst</span>.<span class="ident">goto</span>, <span class="ident">at_next</span>);
                }
                <span class="bool-val">false</span>
            }
            <span class="ident">Bytes</span>(<span class="kw-2">ref</span> <span class="ident">inst</span>) <span class="op">=&gt;</span> {
                <span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">b</span>) <span class="op">=</span> <span class="ident">at</span>.<span class="ident">byte</span>() {
                    <span class="kw">if</span> <span class="ident">inst</span>.<span class="ident">matches</span>(<span class="ident">b</span>) {
                        <span class="self">self</span>.<span class="ident">add</span>(<span class="ident">nlist</span>, <span class="ident">thread_caps</span>, <span class="ident">inst</span>.<span class="ident">goto</span>, <span class="ident">at_next</span>);
                    }
                }
                <span class="bool-val">false</span>
            }
            <span class="ident">EmptyLook</span>(_) <span class="op">|</span> <span class="ident">Save</span>(_) <span class="op">|</span> <span class="ident">Split</span>(_) <span class="op">=&gt;</span> <span class="bool-val">false</span>,
        }
    }

    <span class="doccomment">/// Follows epsilon transitions and adds them for processing to nlist,</span>
    <span class="doccomment">/// starting at and including ip.</span>
    <span class="kw">fn</span> <span class="ident">add</span>(
        <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>,
        <span class="ident">nlist</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">Threads</span>,
        <span class="ident">thread_caps</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="prelude-ty">Option</span><span class="op">&lt;</span><span class="ident">usize</span><span class="op">&gt;</span>],
        <span class="ident">ip</span>: <span class="ident">usize</span>,
        <span class="ident">at</span>: <span class="ident">InputAt</span>,
    ) {
        <span class="self">self</span>.<span class="ident">stack</span>.<span class="ident">push</span>(<span class="ident">FollowEpsilon</span>::<span class="ident">IP</span>(<span class="ident">ip</span>));
        <span class="kw">while</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">frame</span>) <span class="op">=</span> <span class="self">self</span>.<span class="ident">stack</span>.<span class="ident">pop</span>() {
            <span class="kw">match</span> <span class="ident">frame</span> {
                <span class="ident">FollowEpsilon</span>::<span class="ident">IP</span>(<span class="ident">ip</span>) <span class="op">=&gt;</span> {
                    <span class="self">self</span>.<span class="ident">add_step</span>(<span class="ident">nlist</span>, <span class="ident">thread_caps</span>, <span class="ident">ip</span>, <span class="ident">at</span>);
                }
                <span class="ident">FollowEpsilon</span>::<span class="ident">Capture</span> { <span class="ident">slot</span>, <span class="ident">pos</span> } <span class="op">=&gt;</span> {
                    <span class="ident">thread_caps</span>[<span class="ident">slot</span>] <span class="op">=</span> <span class="ident">pos</span>;
                }
            }
        }
    }

    <span class="doccomment">/// A helper function for add that avoids excessive pushing to the stack.</span>
    <span class="kw">fn</span> <span class="ident">add_step</span>(
        <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>,
        <span class="ident">nlist</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">Threads</span>,
        <span class="ident">thread_caps</span>: <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="prelude-ty">Option</span><span class="op">&lt;</span><span class="ident">usize</span><span class="op">&gt;</span>],
        <span class="kw-2">mut</span> <span class="ident">ip</span>: <span class="ident">usize</span>,
        <span class="ident">at</span>: <span class="ident">InputAt</span>,
    ) {
        <span class="comment">// Instead of pushing and popping to the stack, we mutate ip as we</span>
        <span class="comment">// traverse the set of states. We only push to the stack when we</span>
        <span class="comment">// absolutely need recursion (restoring captures or following a</span>
        <span class="comment">// branch).</span>
        <span class="kw">use</span> <span class="ident">prog</span>::<span class="ident">Inst</span>::<span class="kw-2">*</span>;
        <span class="kw">loop</span> {
            <span class="comment">// Don&#39;t visit states we&#39;ve already added.</span>
            <span class="kw">if</span> <span class="ident">nlist</span>.<span class="ident">set</span>.<span class="ident">contains</span>(<span class="ident">ip</span>) {
                <span class="kw">return</span>;
            }
            <span class="ident">nlist</span>.<span class="ident">set</span>.<span class="ident">insert</span>(<span class="ident">ip</span>);
            <span class="kw">match</span> <span class="self">self</span>.<span class="ident">prog</span>[<span class="ident">ip</span>] {
                <span class="ident">EmptyLook</span>(<span class="kw-2">ref</span> <span class="ident">inst</span>) <span class="op">=&gt;</span> {
                    <span class="kw">if</span> <span class="self">self</span>.<span class="ident">input</span>.<span class="ident">is_empty_match</span>(<span class="ident">at</span>, <span class="ident">inst</span>) {
                        <span class="ident">ip</span> <span class="op">=</span> <span class="ident">inst</span>.<span class="ident">goto</span>;
                    }
                }
                <span class="ident">Save</span>(<span class="kw-2">ref</span> <span class="ident">inst</span>) <span class="op">=&gt;</span> {
                    <span class="kw">if</span> <span class="ident">inst</span>.<span class="ident">slot</span> <span class="op">&lt;</span> <span class="ident">thread_caps</span>.<span class="ident">len</span>() {
                        <span class="self">self</span>.<span class="ident">stack</span>.<span class="ident">push</span>(<span class="ident">FollowEpsilon</span>::<span class="ident">Capture</span> {
                            <span class="ident">slot</span>: <span class="ident">inst</span>.<span class="ident">slot</span>,
                            <span class="ident">pos</span>: <span class="ident">thread_caps</span>[<span class="ident">inst</span>.<span class="ident">slot</span>],
                        });
                        <span class="ident">thread_caps</span>[<span class="ident">inst</span>.<span class="ident">slot</span>] <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">at</span>.<span class="ident">pos</span>());
                    }
                    <span class="ident">ip</span> <span class="op">=</span> <span class="ident">inst</span>.<span class="ident">goto</span>;
                }
                <span class="ident">Split</span>(<span class="kw-2">ref</span> <span class="ident">inst</span>) <span class="op">=&gt;</span> {
                    <span class="self">self</span>.<span class="ident">stack</span>.<span class="ident">push</span>(<span class="ident">FollowEpsilon</span>::<span class="ident">IP</span>(<span class="ident">inst</span>.<span class="ident">goto2</span>));
                    <span class="ident">ip</span> <span class="op">=</span> <span class="ident">inst</span>.<span class="ident">goto1</span>;
                }
                <span class="ident">Match</span>(_) <span class="op">|</span> <span class="ident">Char</span>(_) <span class="op">|</span> <span class="ident">Ranges</span>(_) <span class="op">|</span> <span class="ident">Bytes</span>(_) <span class="op">=&gt;</span> {
                    <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">t</span> <span class="op">=</span> <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">nlist</span>.<span class="ident">caps</span>(<span class="ident">ip</span>);
                    <span class="kw">for</span> (<span class="ident">slot</span>, <span class="ident">val</span>) <span class="kw">in</span> <span class="ident">t</span>.<span class="ident">iter_mut</span>().<span class="ident">zip</span>(<span class="ident">thread_caps</span>.<span class="ident">iter</span>()) {
                        <span class="kw-2">*</span><span class="ident">slot</span> <span class="op">=</span> <span class="kw-2">*</span><span class="ident">val</span>;
                    }
                    <span class="kw">return</span>;
                }
            }
        }
    }
}

<span class="kw">impl</span> <span class="ident">Threads</span> {
    <span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-&gt;</span> <span class="self">Self</span> {
        <span class="ident">Threads</span> {
            <span class="ident">set</span>: <span class="ident">SparseSet</span>::<span class="ident">new</span>(<span class="number">0</span>),
            <span class="ident">caps</span>: <span class="macro">vec</span><span class="macro">!</span>[],
            <span class="ident">slots_per_thread</span>: <span class="number">0</span>,
        }
    }

    <span class="kw">fn</span> <span class="ident">resize</span>(<span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">num_insts</span>: <span class="ident">usize</span>, <span class="ident">ncaps</span>: <span class="ident">usize</span>) {
        <span class="kw">if</span> <span class="ident">num_insts</span> <span class="op">==</span> <span class="self">self</span>.<span class="ident">set</span>.<span class="ident">capacity</span>() {
            <span class="kw">return</span>;
        }
        <span class="self">self</span>.<span class="ident">slots_per_thread</span> <span class="op">=</span> <span class="ident">ncaps</span> <span class="op">*</span> <span class="number">2</span>;
        <span class="self">self</span>.<span class="ident">set</span> <span class="op">=</span> <span class="ident">SparseSet</span>::<span class="ident">new</span>(<span class="ident">num_insts</span>);
        <span class="self">self</span>.<span class="ident">caps</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="prelude-val">None</span>; <span class="self">self</span>.<span class="ident">slots_per_thread</span> <span class="op">*</span> <span class="ident">num_insts</span>];
    }

    <span class="kw">fn</span> <span class="ident">caps</span>(<span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">pc</span>: <span class="ident">usize</span>) <span class="op">-&gt;</span> <span class="kw-2">&amp;</span><span class="kw-2">mut</span> [<span class="prelude-ty">Option</span><span class="op">&lt;</span><span class="ident">usize</span><span class="op">&gt;</span>] {
        <span class="kw">let</span> <span class="ident">i</span> <span class="op">=</span> <span class="ident">pc</span> <span class="op">*</span> <span class="self">self</span>.<span class="ident">slots_per_thread</span>;
        <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>.<span class="ident">caps</span>[<span class="ident">i</span>..<span class="ident">i</span> <span class="op">+</span> <span class="self">self</span>.<span class="ident">slots_per_thread</span>]
    }
}
</pre>
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